Planet Quality does indeed hardcap the target population of a planet.
You can read out from the list at the bottom of this link
https://www.galciv.wikia.com/wiki/Population
how a specific PQ influences that. The wikia is quite outdated and alot of infos given in there isn't accurate anymore, but that specific info is still true - even in TotA.
edit:
and BTW, it only limits its own ability to grow more people. If you ferry population manually to such a planet, it will be able to cross these caps and sustain more people if just enough food happens to be there. If not, all excess people will starve at the beginning of a new turn.
